The present invention relates to single-ended electrostatic DC linear particle accelerators. Such accelerators are well known and have been commercially available for more than 50 years to generate MeV electrons and ions. The ease with which the particle energy can be varied over a large range covering several tens of keV up to several tens of MeV, its unparalleled sharp energy definition and beam quality and their relative simple operating principle are the main reasons for their continuing widespread use today. The early accelerators were built in vessels that contained a pressurized gas to isolate the high voltage DC potential. A moving belt continuously transports charge that is sprayed onto its surface towards the terminal, thereby maintaining it at a high voltage potential. These belt driven DC linear electrostatic accelerators are named after their inventor, R. J. Van de Graaff and have limited current capability of typically less than ˜1 mA.
The beam current capability of the MeV DC linear accelerators was increased by several mA by changing the mechanical belt-driven high-voltage power supply by an electronic power supply. Probably the most successful example of such a pure electronic power supply that is applied for megavolt DC linear accelerators is the so-called Dynamitron power supply. Dynamitron-type power supplies are often referred to as parallel-coupled multiplier cascades to indicate their resemblance with today's standard and widespread approach of generating high voltage by serial-coupled multiplier cascades. In conjunction with accelerators serial-coupled multiplier cascaded high-voltage power supplies are often referred to as Cockroft-Walton type power supplies after their inventors J. D. Cockcroft and E. T. S. Walton.
In the case of electron accelerators the ongoing developments of Dynamitrons led to very powerful and high-current machines. Today many Dynamitron-based electron accelerators routinely provide electron beam intensities of several tens of mA and beam powers in excess of 100 kW to serve diverse industrial applications.
In spite of the growing demand by various applications and substantial effort, early high expectations that the availability of high-current DC power supplies and high-intensity ion sources would lead to the availability of several MeV ion beams at tens of mA intensity, never really matured. Examples of these applications include research in astrophysics and cancer therapy. Today, there is an even broader range of applications that would benefit from high-intensity ion beams of H, D or He, including cancer therapy, of which BNCT may be the best example, cyclotron injection, silicon cleaving for e.g. solar cell production, ion implantation in semiconductor devices and NRA for e.g. the detection of explosives.
In short, the reason that the progress in increasing beam current came to a halt can be explained as follows. The increase in primary beam current from the ion source inevitably resulted in the release of more neutral gas from these sources. The neutral gas from the ion source will increase the vacuum pressure inside the acceleration tube that accelerates the primary ion beam. Inside this acceleration tube the interaction of the primary ion beam with neutral gas atoms or molecules will result in several undesirable effects.
First of all, ionization of the neutral gas creates charged particles (ions and electrons) within the acceleration tube and these charged particles will be accelerated by the electrostatic field in the tube. The charged particles in turn will end up on the electrodes of the tube which will upset its field distribution. This in turn will affect the stability and voltage holding capability of the acceleration tube, possibly resulting in a full breakdown of the high voltage.
Secondly, scattering of primary particles on the neutral gas atoms will change their direction within the acceleration tube so that a part of the primary ions will end up on the electrodes of the acceleration tube. This is a second contribution to the reduced voltage holding capability of the acceleration tube.
These obstacles that were limiting the beam current capability are long understood and well described. See for example: US application # US 2010/0033115 and references therein.
Further references are:
    B. Cleff, W.-H. Schulte, H. Schulze, W. Terlau, R. Koudijs, P. Dubbelman and H. J. Peters, A new 2 MV single-ended ion accelerator for ion implantation, Nucl. Instr. and Meth. in Phys. Res. B6 (1985) 46-50    A. Gottdang, D. J. W. Mous and R. G. Haitsma, The novel HVEE 5 MV Tandetron™ Nucl. Instr. and Meth. in Phys. Res. B190 (2002) 177-182
The understanding of the physical phenomena that hampered the increase of ion beam current motivated the design of new accelerators that addressed the underlying problems. These designs include the incorporation of a vacuum pump and a vacuum restriction in the high-voltage terminal, mass analysis before acceleration to ensure that only the ions of interest are accelerated and the application of ion sources with high ionization efficiency to optimize the ratio between the primary ion beam current and the release of neutral gas. There are many examples of DC linear accelerators that are equipped with a vacuum pump and mass analysis inside the high-voltage terminal, see e.g.: B. Cleff, W.-H. Schulte, H. Schulze, W. Terlau, R. Koudijs, P. Dubbelman and H. J. Peters, A new 2 MV single-ended ion accelerator for ion implantation, Nucl. Instr. and Meth. in Phys. Res. B6 (1985) 46-50 and the earlier mentioned US application.
Apart from reasons with regard to technical functionality, it is a practical shortcoming of a configuration that has a vacuum pump in the terminal in that it requires periodic regeneration of the accumulated gas, which is time-consuming and results in system downtime.
In spite of the efforts described above, a clear breakthrough towards currents of tens of mA has not been convincingly demonstrated. Such a breakthrough would widen the field of applications for DC linear accelerators in many directions that are mentioned before. As an example, the availability of a 2-3 MeV proton accelerator system with a beam current capability of roughly 20 mA would pave the road towards the clinical application of Boron Neutron Capture Therapy (BNCT) since such high beam current brings the duration of the treatment within acceptable limits. It is believed that the reduction of the vacuum pressure inside the acceleration tube is the key towards higher currents.